Method of bending of flat tubes for heat exchangers and bent flat tube

ABSTRACT

A heat exchanger, bent flat tube and method for bending of flat tubes for heat exchangers. The flat tube is initially formed as a straight flat tube and is bent in its central portion about a side end by an angle of about 180°. In bending the flat tube, the overall cross-sectional area and height of the flat tube is maintained in the bent central portion. Additionally, the top and bottoms side walls of the flat tube are maintained planar and parallel with one another. When provided in a heat exchanger with other bent flat tubes, fins are located between adjacent flat tubes and are provided in the curved central portion. The bent flat tubes also connect to a header located on one side of the heat exchanger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of bending flat tubes of heat exchangers. whereby the flat tube is provided with an inflow portion, a curved portion, and a return flow portion, all of which cooperate to transport refrigerant therethrough. The heat exchanger itself includes a header with a distributor and a collector to which at least two flat tubes, with fins located therebetween, are connected at both ends via the inflow portion and the return flow portion. The flat tubes may be CO₂ pressure tubes, which may further be configured as high-pressure or low-pressure tubes.

2. Related Technology

Problems exist, as will become readily apparent, in bending flat tubes in a direction generally corresponding to their width. The current method of bending flat tubes involves first twisting the flat tube (by an angle of about 90°) about its longitudinal axis in a central portion of its total length. Next, in a location beyond the twist, the flat tube is bent (by an angle of about 180°) about an axis that is transverse to the longitudinal axis of the flat tube. Finally, in a location beyond the prior bend, the flat tube is again twisted (by and angle of about 90°) about its longitudinal axis. This results in a flat tube having and inflow portion and a return flow portion that are side by side or parallel with one another and which are co-planar.

Because of the two twists, in the bend region, the flat tube leaves a fin plane (the surface to which a fin can be attached to the side ends of the tube) having a width of only, for example, about 15 millimeters. Therefore, in this area of the flat tube, the side walls of the flat tube cannot be completely utilized for heat transmission.

Another problem with the current method is that due to design, the possibility does not exist to utilize the passed cross-sectional area completely.

A flat tube heat exchanger with a collector is described in DE 39 36 109 A1. The heat exchanger is provided with a plurality of flat tubes in a stacked arrangement one over another and provided with corrugated fins between adjacently stacked flat tubes. Each flat tube has an inflow portion and a return flow portion connected by a return bend or curved portion. Thus, both the inlet to the flat tube and the outlet from the flat tube are provided on the same side of the heat exchanger. The inflow portion and the return flow portion are rectilinear and both taper at their ends with respect to their widths. Also the curved portion has at both its faces a cross-section tapered, with respect to its width, with the tapered cross sections of the curved portion being located at the tapered cross-sections of the inflow portion and the return flow portion. A problem with this construction is that the taper at both ends of the connection of the curved portion of the flat tube reduces the refrigerant flow rate and hence the heat transmission capacity.

A flat tube with a reversing arc portion (and a heat exchanger configured based on such a flat tube) is described in DE 103 06 848 A1. In this construction the flat tube is bent such that both planar tube portions of the flat tube, adjacent to the arc portion, longitudinally extend in opposing passage directions having longitudinal axes offset to each other, at least in the lateral direction. The reversing arc portion is configured such that a main bending axis extends in a plane that is parallel to the flat tube plane and extends at a given angle relative to the tube longitudinal extension, whereby the flat tube plane is defined by the longitudinal and lateral extensions of the flat tube.

In EP 1 036 296 B1, flat tubes are described for a heat exchanger tube block that have at least one reversing arc portion. Each flat tube is bent such that both its adjacent planar tube portions extend with opposing passage directions and longitudinal axes being offset with each other, at least in lateral direction. In the reversing arc portion, the flat tube lateral axis of the flat tube includes an angle of 45° (maximum) to a plane parallel to the longitudinal and lateral directions, vertical to a stacking direction.

A problem with the reversing arc portions is that the cross-section of the flat tube, in the reversing arc portions, is reduced so that the refrigerant flow rate is reduced. In addition, there are thinner wall thicknesses in the reversing arc portions, and the fin configuration is not continuous over the complete extension of the flat tube.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method for bending flat tubes to be used in heat exchangers. The bent flat tube is configured so that after the bending process, the cross-sectional area and the wall thickness in the reversing arc portion are not reduced. Also, between the two co-planar and offset portions of the flat tubes, a continuous fin configuration is ensured.

The method for bending a flat tube according to the present invention generally includes the steps of bending the flat tube via the central portion thereof, about a side end of the flat tube, by an angle of 180° relative to the longitudinal axis; processing the flat tube in the bending radius (the reversing arc portion); and/or making the flat tube larger/longer in the bending radius during bending, whereby the total cross-sectional area and the thickness of the wall of the flat tube are maintained in the bending radius and whereby the tube top surface and the tube bottom surface of the flat tube, also in the reversing arc portion, are planar and parallel to each other.

During the bending process, the flat tube is secured in its cross-sectional area. whereby the flat tube is bent on a mandrel using a bending device. During bending the flat tube may be overbent because of backspring of the flat tube. Further, it is also possible to cold bend the flat tube, i.e. at room temperature, (namely at approximately 20° C.), whereby, it is useful to change the pre-set bending parameters. Bending can be carried out in a temperature range between room temperature and a maximum temperature of 150° C. The deformation of the inner cross-sectional area can be influenced, to a high degree, by the pre-set bending speed.

One advantage of a so bent flat tube is that the heat transferring capacity of the flat tube is increased. This increase occurs because of the parallelity of the planar tube top surface and planar tube bottom surface are maintained and, as such, the tube wall can continuously have fins over its length and can be completely used for heat transmission.

In one aspect of a flat tube manufactured and bent using the method of the invention, the central portion of the flat tube is provided with an in-plane bend of an angle of about 160° to 180°. In the whole reversing arc portion, the cross-sectional area and the thickness of the flat tube are substantially constant and the tube top surface in continuously aligned parallel to the tube bottom surface. As such, the total cross-sectional area of the flat tube can be utilized for heat transfer in the reversing arc portion.

A highly efficient, three-fluid, compact heat exchanger can be provided with a header to which the bent flat tubes are filled with the end regions of the inflow portion and the return flow portion in distributor and collector portions of the header.

Particularly, the bent flat tubes can be used in combined CO₂/air/glycol heat exchangers. which are equipped with a header for the CO₂ flow.

When designing a heat exchanger with a given flow/flow rate of the CO₂ refrigerant flow, a certain tube geometry is provided. Bending of the wall radii at the long side of the tubes allows the flat surface within the central portion to be provided with fins so that a fin can be continuously arranged between the central portions of neighboring flat tubes.

Due to the flat bending process, the invention makes it possible to optimally utilize the total cross-sectional area. Therefore, due to the design, the efficiency of the heat transmission can be enhanced. The invention also makes possible to achieve thermodynamic advantages when soldering or brazing the flat tubes of the fins.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by means of an example of embodiment with reference to several drawings.

FIG. 1 is a schematic representation of a bent flat tube embodying the principles of the present invention;

FIG. 2 is a cross-section,. taken generally along line A-A in FIG. 1, of the bent flat tube seen in FIG. 1;

FIG. 3 is a cross-sectional view of a portion of a heat exchanger embodying the bent flat tubes of the present invention; and

FIG. 4 is a partial side view of a heat exchanger incorporating the bent flat tubes in accordance with the principles of the present invention.

DETAILED DESCRIPTION

In the following, reference is made to FIGS. 1 and 2 in common.

As seen in FIG. 1, a bent flat tube 1 is provided with an inflow portion 2, a curved central portion 3 and a return flow portion 4, for the transport of a refrigerant for heat transmission. A heat exchanger, seen in FIG. 4, is provided with a header 20 to which at least two bent flat tubes 1, with fins 22 arranged therebetween, are connected each at both ends with the inflow portion 2 and the return flow portion 4.

According to the invention, the central portion 3 of the flat tube 1 is provided with a plane bend 5, a reversing arc portion, at an angle of 160° to 180°, whereby in the central portion 3 the overall cross-sectional area 6 and the thickness of the wall 7 of the flat tube 1 are configured substantially constant, and the top side wall surface 15 is continuously aligned parallel to the bottom side wall surface 14 in the central portion 3.

As refrigerant a triple combination of carbon dioxide-CO₂—, air and glycol can be used.

For example, the flat tube 1 can be a singular multi-channel tube with a rectangular overall cross-sectional area 6 of 6.0 mm to 8.5 mm (as to the one side (width) of the rectangle) 1.5 mm to 2.8 mm (as to the other side (height) of the rectangle), whereby the tube 1 is centrally bent by an angle of 180° in lateral direction (relative to the longitudinal axis 11 of the tube 1). The tube 1 has as connections for a header a port entrance 8 for the inflow portion 2 and a port exit 9 for the return flow portion 4, whereby both ports 8, 9 are connected to the distributor and collector portions of a header 20 via soldering or brazing.

After the bending process, it is possible to continuously utilize the side wall top surface 14 and bottom side wall surface 15 in the plane bend 5 as fin connection surfaces between adjacent tubes 1.

The method for bending flat tubes 1 for heat exchangers, includes the steps: bending the flat tube 1 at the narrow longitudinal side 10 in the central portion 3 by an angle 180° to the longitudinal axis 11; processing the flat tube 1 in the inner bending radius 12; and/or making the flat tube 1 larger/longer in the outer bending radius 13 during bending, whereby the overall cross-sectional area 6 and the thickness of the wall 7 of the flat tube 1 are generally maintained in the bending radius 12, 13 and whereby the top side wall surface 15 and the bottom side wall surface 14 of the flat tube 1 remain configured co-planar and parallel to each other.

During bending, the over thickness/height of the wall 7 of the flat tube 1 can be set to remain constant. During the bending process, the flat tube 1 is secured via its cross-sectional area 6. and the flat tube 1 is bent on a mandrel using a bending device. During bending the flat tube 1 may be overbent because of backspring of the flat tube 1. The bending process can be executed at room temperature, for example 20° C. at a maximum operational temperature of 150° C. or at a temperature in between.

In a multiflow CO₂/air/glycol heat exchanger the bent CO₂-flat tubes 1 are secured via the ports 8, 9 at a common side of the heat exchanger. Preferably, the flat tubes 1 according to the invention are made of AA3103 or AA6003 or other suitable materials.

During bending, the invention enables avoids compression, as well as deformation, as to the material thickness of the wall 7. Deformation of the inner cross-sectional area can be influenced to a high extent by a pre-given bending speed, whereby it may be beneficial to change the bending parameters based on the material and design criteria of the specific design.

As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims. 

1. A method for bending of flat tubes for heat exchangers, whereby the flat tube is provided with an inflow portion. a curved central portion, and a return flow portion for a refrigerant co-planar therewith, the heat exchanger being provided with a header having separate portions to distribute and collect the refrigerant. at least two flat tubes stacked in relation to one another being connected to the header and fins being located between the flat tubes, the method comprising the following steps: forming a straight flat tube having a longitudinal axis; bending the flat tube at a central portion about a side end of the flat tube by an angle of about 180° relative to the longitudinal axis; processing the flat tube at an inner bending radius; making the flat tube longer at an outer bending radius during bending; generally maintaining the overall total cross-sectional area and the thickness of the flat tube in the curved central portion, whereby the top side wall surface and the bottom side wall surface of the flat tube are maintained planar and aligned parallel to each other.
 2. The method of claim 1 wherein bending is carried out in a temperature range of between about 20° C. and 150° C.
 3. The method of claim 1 wherein the step of bending the flat tube includes the step of securing the cross-sectional area of the flat tube.
 4. The method of claim 1 wherein the step of bending the flat tube includes the step of securing the flat tube relative to a mandrel and using a bending device.
 5. The method of claim 1 wherein the step of bending the flat tube includes the step of overbending the flat tube because of the backspring of the flat tube.
 6. A bent flat tube for a fin and tube heat exchanger, the bent flat tube comprising: a flat tube defining a longitudinal axis and including an inflow portion, a curved central portion and a return flow portion configured for a refrigerant to flow therethrough, the central portion of the flat tube having a plane bend at an angle in the range of 160° to 180°, whereby throughout the curved central portion a substantially constant overall cross-sectional area and thickness of flat tube exists, the top side wall surface of the flat tube being planar and parallel to the bottom side wall surface in the curved central portion.
 7. The bent flat tube of claim 6 incorporated in a heat exchanger.
 8. The invention of claim 7 wherein the heat exchanger includes a plurality of bent flat tubes stacked adjacently on top of one another.
 9. The invention of claim 8 wherein the plurality of bent flat tubes are parallel to one another and at least one fin is located between adjacent ones of the bent flat tubes.
 10. The invention of claim 9 wherein the at least one fin is provided in the curved central portion.
 11. The invention of claim 7 wherein the heat exchanger includes a header on one side thereof and the inflow portions and return flow portions of the plurality of flat tubes are connected to the header.
 12. The invention of claim 7 wherein the bent flat tube defines a plurality of passageways therethrough. 